The present invention relates to vehicle wheel alignment, and more particularly to calibrating vehicle wheel alignment systems which employ electrical, electro-optical, and/or electro-mechanical transducers to measure the wheel alignment of the vehicle.
Reissued U.S. Pat. No. Re 33,144 to Hunter and January and U.S. Pat. No. 4,319,838 to Grossman and January each describe a wheel alignment system which uses electro-optical transducers to determine the toe alignment angles of a vehicle. FIG. 2 of each of these patents shows six angle transducers carried by support assemblies which are mounted to the vehicle wheels. FIG. 4 of the U.S. Pat. No. Re 33,144 patent and FIG. 9 of the U.S. Pat. No.4,319,838 patent show the geometry of this arrangement and illustrate the six angles which are directly measured. These patents further describe (see U.S. Pat. No. Re 33,144 col. 7 lines 26-39, and U.S. Pat. No. 4,319,838 col. 8 line 63 to col. 9 line 12) how the toe alignment angles are computed from the angles directly measured by the angle transducers.
U.S. Pat. No. 4,879,670 to Colarelli (the disclosure of which is incorporated herein by reference) describes a gravity-referenced inclinometer for use in measuring wheel alignment characteristics such as camber. FIG. 1A of the U.S. Pat. No. 4,879,670 patent shows such an inclinometer carried by a support assembly which is mounted to a vehicle wheel.
Equipment of this general type has been used world-wide for many years. While the angle measurement transducers used in such systems are built using many different technologies and methods, all share the common requirement that they must be calibrated to produce accurate results. Each transducer must be "zero calibrated" in that it must be adjusted to produce a measurement which means "zero" when it is placed in a position in which the angle to be measured is defined to be "zero". Each transducer must also be "range calibrated" in that it must be adjusted to produce a change in its measurement due to a change in its angular position such that the change in measurement equals the change in the angular position.
Calibration is extremely important because the quality of alignment measurements cannot be greater than the quality of calibration of the instruments used to make those measurements. The quality of calibration is limited by the apparatus and method used to achieve the calibration. In short, a high quality wheel alignment system can be rendered useless by a low quality calibration. Although the use of a calibration fixture with an associated calibration procedure is old and widely practiced in the art, many of these fixtures and/or procedures have inherent limitations which cause them to produce inaccurate or unreliable calibrations.
Calibration can be performed in two general ways. One way is to physically adjust the mechanical and/or electrical components of the transducers, for example by changing the focus of lenses or by adjusting potentiometers. The second way is to measure the signals produced by the transducers during the calibration procedure, and then, via computer software, compute and store calibration coefficients which allow the software to compute accurate values from the transducer signals. This second way has been widely practiced for more than a decade, and is well understood in the art.
It is reasonable to expect an alignment system to be calibrated during the manufacturing process. Such systems also need to be calibrated at periodic intervals to correct the effects of general wear and tear and when components are repaired or replaced. Such "field calibrations" must be performed at the alignment work site to avoid the down time required by shipping such systems to a factory or other repair site.